Capabilities and configuration

A structure describing the capabilities of a session. This record
serves dual roles.

It's used to specify the desired capabilities for a session before
it's created. In this usage, fields that are set to Nothing indicate
that we have no preference for that capability.

When returned by getCaps, it's used to
describe the actual capabilities given to us by the WebDriver
server. Here a value of Nothing indicates that the server doesn't
support the capability. Thus, for Maybe Bool fields, both Nothing and
Just False indicate a lack of support for the desired capability.

Default capabilities. This is the same as the Default instance, but with
a more specific type. By default we use Firefox of an unspecified version
with default settings on whatever platform is available.
All Maybe Bool capabilities are set to Nothing (no preference).